1. Field of the Invention
The present invention relates generally to packing boxes and more particularly to a packing case made of a sheet of foamed or expanded thermoplastic material or a sheet of thermoplastic corrugated board and an apparatus for producing the same.
2. Description of the Prior Art
It generally is comparatively easy to produce a square box without a lid from a sheet of plastic material, however, conventional boxes of this character which are commonly known in the art do not have enough strength against torsional forces and are therefore easily deformed by only slight external forces. Further, especially in the case of bulky boxes, they are liable to slip from one's grasp because of the physical nature of the material and they are therefore inconvenient to carry. Also, in such boxes having no lids and being common in shape, there is the problem in storing in that the boxes can not be piled or stacked very high because the stacking surfaces thereof are not planar. In order to prevent deformation of the boxes, which is one of the principal defects in the case of such conventional types, reinforcing members or overlapping areas to which paste is applied may be provided. However, these steps are not advisable, because they not only cause the thickness of the side walls of the box to be uneven, but they also require the use of additional materials for making the reinforcing members or the overlapping areas. Further, these steps still do not make it possible to stack such boxes one above another.